While the multinational search continues for the whereabouts of the aircraft which carried 239 people, the FBI — citing at least three American citizens who were passengers — will investigate the circumstances surrounding the jet's strange disappearance from radar screens in Vietnam airspace as it traveled at 35,000 feet.

The Times has more:

U.S. officials said they are looking at whether this could be terrorism, as they would with any plane crash until proved otherwise. Though two passengers apparently used stolen passports, “there is no indication this is a terrorist attack; stolen passports are certainly not indicative of a terrorist attack,” a senior counter terrorism official said.

The official said there was "no evidence" of terrorism thus far. Law enforcement officials were not authorized to speak publicly.